Did you mean by adding the son DL as a member of the parent DL, you would like to have all members in the son DL to be able to send as the parent DL?
If I misunderstood the question, please feel free to correct me.
If this is the case, please note that you cannot add a whole DL as delegate.
Instead you need to manually add all members in the son DL as Send As delegates of the parent DL.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.